In recent years, PMs (particulate matters, hereinafter, referred to as particulates) contained in exhaust gases discharged from inner combustion engines of vehicles such as buses, trucks or the like and construction machines and the like have raised serious problems since those particulates are harmful to the environment and the human body.
There have been proposed various ceramic filters which allow exhaust gases to pass through porous ceramics and collect particulates in the exhaust gases to purify the exhaust gases.
Conventionally, with respect to the ceramic filter of this type, there have been proposed honeycomb filters which have a structure in that a number of through holes are placed in parallel with one another in one direction and wall portion that separate the through holes from each other are allowed to function as filters. In other words, each of the through holes formed in the ceramic filter is sealed with a plug at either of ends of its exhaust gas inlet side and outlet side so that exhaust gases that have entered one through hole are discharged from another through hole after having always passed through each wall portion that separates the through holes.
An exhaust gas purifying device has a structure in that the honeycomb filter having the above-mentioned arrangement is provided in an exhaust gas passage of an internal combustion engine, and when particulates in exhaust gases discharged from the internal combustion engine are made to pass through the honeycomb filter, the particulates are captured by the wall portion so that the exhaust gases are purified.
As such a purifying process for exhaust gases progresses, particulates are gradually accumulated on the partition wall that separates the through holes of the ceramic filter to cause clogging and the subsequent interruption in gas permeability. For this reason, the above-mentioned ceramic filter needs to be subjected to a regenerating process regularly by allowing gases heated by a heating means such as a heater or the like to flow through the through hole so as to burn and remove the particulates that cause the clogging.
In such a regenerating process of the conventional honeycomb filter, however, it is difficult to completely burn and remove particulates accumulated on the partition wall, and the regenerating process of the honeycomb filter causes residual ash components (ashes) derived from metal components of the particulates on the partition wall. Normally, these ashes remain over the almost entire face of the wall portion in an even state, and the conventional honeycomb filter causes a difficulty in separating the ashes from the wall portion, and can hardly move the ashes inside the through hole by using gases that enter the through hole in the regenerating process of the honeycomb filter (see FIGS. 9(a) to 9(c)).
Here, FIG. 9(a) is a cross-sectional photograph that shows a cross-section of such a honeycomb filter taken in parallel with the length direction, FIG. 9(b) shows partially enlarged cross-sectional photographs that show cross-sections of the honeycomb filter of FIG. 9(a) in the vicinity of the exhaust gas inlet side, the center portion and the exhaust gas outlet side, and FIG. 9(c) shows partially enlarged cross-sectional photographs perpendicular to the length direction in the vicinity of the exhaust gas inlet side, the center portion and the exhaust gas outlet side of the honeycomb filter. In the above-mentioned honeycomb filter, it is confirmed that white ashes remain over the almost entire face of the partition wall.
For this reason, in the case where particulate collecting process is carried out by using the exhaust gas purifying device using the conventional honeycomb filter, since the regenerating efficiency is low in the honeycomb filter regenerating process, the pressure loss is quickly increased to cause necessity of frequent regenerating processes of the honeycomb filter.
When the collecting process and regenerating process of particulates are repeatedly carried out, the amount of ashes remaining in an even state over the almost entire face of the partition wall of the honeycomb filter quickly increases to cause clogging in the partition wall due to the ashes. In this case, since the initial pressure loss after the regenerating process of the honeycomb filter due to the residual ashes becomes high, it becomes necessary to frequently carry out washing processes for removing the ashes.
In the washing processes for removing the ashes, normally, the above-mentioned honeycomb filter needs to be detached from the exhaust gas purifying device provided in the exhaust gas passage in the internal combustion engine, and subjected to a washing process with water, a chemical treatment and the like; therefore, it is not possible to continuously use the conventional exhaust gas purifying device for a long time.